[openapi to tsp] Fix accept header logic

[catalinaperalta]

Given the following swagger definition:

        "produces": [
          "application/vnd.microsoft.appconfig.kv+json",
          "application/problem+json"
        ],

The converter outputs the following typespec code where all of the supported content types are bundled into 1 string:

accept: "application/vnd.microsoft.appconfig.kv+json, application/problem+json";

The correct code should represent each content type individually:

accept: "application/vnd.microsoft.appconfig.kv+json" | "application/problem+json";

cc @joheredi